The present invention relates to determining the average paper fiber area density, particularly to an optical method for determining the average paper fiber area density, and more particularly to an optical method for determining the average paper fiber area density (weight per unit area) from the intensity of transmitted and scattered light at a non-absorbed wavelength using a two-wavelength measurement.
The average paper fiber area density (weight per unit area) is an important parameter in the determination of paper characteristics. It is often calculated from the ratio of its dry weight and its total area. This measuring process is quite laborious and can be done only in well equipped laboratories.
Also, the measurement of the water content of paper is of great concern. Recently, an optical measuring technique for measuring water content was developed. See U.S. Pat. No. 4,840,706, issued Jun. 20, 1989 to N. F. Campbell, which was designed for point detection, wherein a modified infra-red scanning gauge was used in measuring the moisture content of a paper-web during manufacturing, utilizing a measurement channel and a reference channel, with the measurement channel preferably being at a wavelength band having a center wavelength of 1.83 micron, and the reference channel preferably being at a wavelength band having a center wavelength of about 1.7 micron. The reference wavelength is remote from the measurement wavelength wherein radiation in the reference wavelength and is substantially unaffected by the moisture in the paper.
As an optical beam transmits through a sheet of paper, which is generally composed of about 20 to 30 multiple layers of cellulose fibers, more or less oriented in some preferred direction, the part of the optical beam which is not absorbed, scatters in all directions. In the forward direction, the intensity distribution of the scattered optical beam generally depends upon dielectric constant, shape, size, orientation of the paper fibers, and the scattered wavelength. In general, it is not evenly distributed. As the number of layers of cellulose fibers increases, scattering processes become more random. However, the forward transmitted optical beam becomes more uniform. Thus, in any sheet of paper, composed of 20 to 30 layers of cellulose fibers, forward scattered optical intensity may well be assumed to be uniform and isotropic. One defines this forward transmitted emission as a diffused emission and it satisfies Lambert's cosine law. See Max Born and Emil Wolf, "Principles of Optics", Fourth Edition, page 181, Chapter 4.8, Photometry and Apertures.
The present invention involves an optical measuring technique or method wherein the average paper fiber area density (weight per unit area) can be directly calculated from the intensity of transmitted, scattered light at a non-absorbed wavelength coupled with the method of using a two-wavelength measurement approach for water content measurement. Also, it has been experimentally shown that it is possible to derive the water percentage per fiber area density from the two-wavelength measurement. Thus, using this invention, paper area density measurement can be obtained from forward transmitted scattered light. In this optical measuring method, the system can be calibrated initially by an optical method: an optical transmitted intensity at 2.1 microns cellulose absorption line is measured and compared with the same scattered, optical transmitted intensity reference used for density measurement in the nearby spectrum region (1.68 microns), where there is no absorption. From the ratio of these two intensities, one can calculate the scattering absorption coefficient at 2.1 microns and calibrate the system. This absorption coefficient for this wavelength is, then, experimentally correlated to the paper fiber area density.